When various processes such as a plasma process are performed on various kinds of substrates such as a semiconductor wafer (hereinafter, simply referred to as a “wafer”), a temperature of a wafer and/or temperatures of components of a plasma processing apparatus are measured in order to perform the process precisely. Recently, there has been proposed a technique related to a low-coherence light interference thermometer for measuring a temperature of a temperature measurement target by irradiating low-coherence light to the temperature measurement target and measuring an interference between reflected light from a front surface and reflected light from a rear surface of the temperature measurement target (see, for example, Patent Document 1).    Patent Document 1: Japanese Patent Laid-open Publication No. 2003-307458
In the conventional temperature measurement technique using the low-coherence light interference thermometer, however, various conditions need to be satisfied. For example, the temperature measurement target needs to be transmissive to a part of measurement light; parallelism between the front surface and the rear surface of the temperature measurement target needs to be high; and the front surface and the rear surface need to be mirror-polished. Besides, there are many restrictions required for the temperature measurement target. Thus, the range of applications of the low-coherence light interference thermometer has not been so wide. Furthermore, a distance between a collimator for irradiating low-coherence light and the temperature measurement target, i.e., an optical path length needs to be set accurately. Even if an actual optical path length is slightly deviated from an appropriate optical path length, accurate temperature measurement cannot be performed. Thus, manipulation for setting the optical path length has been complicated and troublesome.